1 . Field of the Invention
The invention relates to a transmission torque estimation unit that estimates a torque that a driving force transmission apparatus is able to transmit.
2 . Description of the Related Art
A driving force transmission apparatus is used, for example, as a coupling that is able to switch the vehicle drive mode between a two-wheel drive mode and a four-wheel drive mode. Japanese Patent Application Publication No. 2012-072892 (JP 2012-072892 A) describes a configuration in which a cam mechanism is provided between a main clutch and an electromagnetic clutch. In such a driving force transmission apparatus, the cam mechanism operates in accordance with the electromagnetic clutch that operates in response to a control signal, thereby changing the engagement state of the main clutch. Such a configuration makes it possible to adjust a torque to be transmitted, according to the control signal.
Whether the control and the design of the driving force transmission apparatus are appropriate is evaluated on the basis of, for example, whether a target transmission torque based on an input torque and a control signal is output from the driving force transmission apparatus. Therefore, it is necessary to accurately figure out a transmission torque that varies depending on the control signal and the state of the driving force transmission apparatus. In contrast to this configuration, for example, modeling in which a clutch is assumed to be formed of two clutch discs has been known. With this modeling, it is possible to generally estimate a transmittable torque, based on a coefficient of friction corresponding to a relative velocity between the clutch discs and a pressing force that acts on each clutch disc.
However, in the driving force transmission apparatus, when a transmission torque with respect to rotational angles of an input shaft and an output shaft is actually measured, the transmission torque is indicated by a complicated hysteresis curve due to the influence of each member, as illustrated in FIG. 6B in JP 2012-072892 A. Therefore, with the aforementioned simple model, a large difference may occur between an estimated torque and an actually transmittable torque.